U.S. Pat. No. Re. 25,770 describes a gas-liquid contacting process for hydrogenating a carbonaceous material which is characterized by an upflow, liquid phase condition of controlled velocity of liquid and gas wherein particulate contact solids are placed in random motion in the liquid without substantial carryover of the solids from the reactor. By such means, it is possible to achieve nearly iso-thermal conditions even with highly exothermic reactions such as hydrogenation and thus obtain optimum contact at optimum temperature conditions. This so-called "ebullated" bed concept has been demonstrated as effective for the liquefaction of coal.
A subsequent U.S. Pat. No. 3,519,555, discloses improvements in the aforementioned patent wherein by controlled reaction zone conditions, yields of liquid of the character of synthetic crude oil can be obtained to the extent of about four barrels per ton of coal on a moisture and ash free (maf) basis.
U.S. Pat. No. 3,519,553 to Johnson et al discloses a coal conversion process utilizing a catalytic ebullated bed reactor for producing a range of vapor and liquid products, and which recycles a high concentration residuum stream to the reaction zone. However, this process also utilizes a suitable wash liquid to assist in the separation of the residuum and solids from the heavy liquids. Further, U.S. Pat. No. 3,540,995 to Wolk et al discloses a coal hydrogenation process wherein the concentrations of liquid residuum and unconverted solids in the reaction zone are maintained within desired ranges by recycling to the reactor certain heavy liquid streams from which solids have been partially removed. Although Wolk et al recycles to the reactor as slurrying oil a combination of heavy distillate and heavy vacuum still bottoms materials, he does not disclose the desirability of recycling certain selected higher liquids which have now been found highly advantageous in improving the yield of desirable 400.degree.-800.degree. F. boiling range liquids.